Why use the key of C#?

Question

I was reading about Brahms, and it was mentioned that he had composed a piece in C# Major. AFAIK, we generally don't use C# Major in jazz or rock music because the key signature has 7 sharps, so it's easier to use its enharmonic key - Db, which only has 5 flats in its key signature. Why would a classical composer like Brahms compose something in C#?

Answer 1

There could be a multitude of reasons ranging from practical choices to personal preference. There might be prevailing orchestrational practices like certain instruments prefer keys with sharps due to technical, performance related issues, intonational considerations or other instrumental idiosyncrasies. It may be part of a larger work, and the harmonic and tonality choices help tie the piece in question to other parts of the composition at large. Or it could even be something as simple as Brahms wanted to write something in C# that day because he hadn't done it in a while or he had some other personal reason. Sometimes it's just because the composer wanted to. Honestly, sometimes composers make bad choices and it could be that the work would have made more sense in Db! It wouldn't be the first time a great composer left people scratching their heads.

Answer 2

There are generally two reasons for using an enharmonic with more sharps/flats than it's equivalent. Often, there are harmonic considerations within the piece that would better lend itself to being written in it's enharmonic equivalent. For instance, if he wanted to use a bIIdim7, it would be notated in C# as D♮ F♮ Ab Cb as opposed to the messier Ebb Gbb Bbbb Dbbb. This is just one example of course, there could be other reasons such as modulations or accidentals in the melody that are notated more cleanly in C#.

The other, more divisive reason is that some composers believe that enharmonic equivalents are perceived differently from one another, whether visually for performers, aurally for listeners, or both. Sharp keys are generally perceived as "brighter" by performers while flat keys are perceived as "darker", which could lead to the performer subconsciously performing the piece according to the key signature's brightness/darkness, possibly against the intentions of the composer. Admittedly there are key differences between the instrumental performance of each, for instance a violinist would typically play C# major starting on 3rd finger on the G string as opposed to 4th finger for Db major even though they're technically the same scale. Theoretically this could lend itself to having minute but notable and consistent intonation/tone differences between the two keys, which further adds to the enharmonic difference belief. I personally think out of the two reasons, Brahms probably considered this one

tl;dr personal preference